An important application of hybrid couplers is the production of directional filters by means of a balanced structure comprising two band-pass filters and two hybrid couplers, and which, commonly known as a double-bridge channel mixer, provides for combining a number of frequencies at the same output.
Currently used UHF-band hybrid couplers comprise quarter-wave coupled lines, and have a band unbalance of about 0.5 dB, which has no particular effect, except noticeably impairing matching of the wide-band transition port of the directional filter. Currently used quarter-wave hybrid couplers, in fact, provide for a maximum theoretical match of 25 dB return loss at the wide-band port. This is decidedly poor when cascade-connection of a number of systems is required, and accumulated reflection inevitably produces unacceptable matching values.
The current trend is to solve the problem by means of complex adjustments and compensating devices, in an attempt to improve matching of only required frequencies.
In any case, the problem involves specific, normally painstaking, adjustments, limits the maximum number of channels that can be combined, and does not permit versatile use of the directional filters, i.e. cascade connection of independent modules. These drawbacks are further compounded in the case of the latest digital television transmission systems (DVB-T and ATSC) which frequently involve inserting a new frequency on an existing radiating system, and, simultaneously, strict transmission filtration (Ref. ETSI EN 300 744 V1.4.1).